1. Field of the Invention
The present invention relates to a solid electrolyte for use in solid electrochemical elements including a solid cell, and more particularly to a lithium ion conductive solid electrolyte whose ion conductive species is lithium ion, and a process for synthesizing the same.
2. Related Art of the Invention
With recent development of portable appliances such as personal computers, portable telephones, etc., a demand for cells as their power source has been considerably increased. Particularly, lithium cells have been extensively studied in various fields as cells capable of giving a high energy density, because lithium is a substance having a small atomic weight and a large ionization energy.
On the other hand, the cells so far used in these fields are based substantially on a liquid electrolyte and thus it has been impossible to eliminate such problems as leakage of liquid electrolyte, etc. To solve these problems, thereby enhancing the reliability and also to make the elements smaller and thinner, attempts for replacing the liquid electrolyte with a solid electrolyte and making an entirely solid cell have been extensively made in various fields.
The above-mentioned lithium, when brought into an abnormal state, has a fear of ignition due to the high energy density. Thus, development of an entirely solid lithium cell using a solid electrolyte made from a non-combustible solid has been desired to ensure the safety of the cell. Lithium halide, lithium nitride, oxy acid salts of lithium, and their derivatives are known as solid electrolytes for use in such a cell. Lithium ion conductive sulfide-based amorphous solid electrolytes such as Li.sub.2 S--SiS.sub.2, Li.sub.2 S--P.sub.2 S.sub.5, Li.sub.2 S--B.sub.2 S.sub.3, etc. are known as solid electrolytes having a particularly high ion conductivity such as more than 10.sup.-4 S/cm.
Ion conductivity of a solid electrolyte, when made into a cell, gives an influence on the internal impedance. For example, a solid cell made from a solid electrolyte having a high ion conductivity has a lower internal impedance than that from a solid electrolyte having a low ion conductivity, and thus can work (i.e. can be charged or discharged) with a larger quantity of electric current. Thus, attempts to increase the ion conductivity have been extensively made in various fields, and it was reported that the above-mentioned sulfide-based solid electrolyte had a high ion conductivity such as 1.times.10.sup.-3 S/cm, when doped with LiI.
However, the sulfide-based solid electrolyte having an increased ion conductivity by doping with LiI still has the following problem. That is, decomposition voltage of LiI is about 2.7 V by thermodynamic calculation, and, thus, when a voltage of more than 2.7 V is applied to LiI, I ions are oxidized at the positive pole. Thus, it is difficult to make cells having a voltage of more than 2.7 V from the solid electrolyte doped with LiI. Dopant compounds other than lithium iodide to improve the ion conductivity generally include, for example, lithium halides such as lithium bromide, lithium chloride, etc., but the lithium halides undergo an oxidation reaction of halide ions, when used as a dopant, with the result that the resulting solid electrolyte has a lowered decomposition voltage.
U.S. Pat. No. 4,585,714 to Akridge relates to a quaternary vitreous lithium conductive electrolyte having a composition, aX, bLi.sub.2 S, Y, Z, where X is P.sub.2 S.sub.5 or SiS.sub.2 ; Y is Li.sub.4 SiO.sub.4, Li.sub.2 CO.sub.3 or Li.sub.2 SiO.sub.3 ; Z is LiI, LiBr, LiCl or LiF. That is, the lithium ion conductor disclosed in the cited reference requires the presence of lithium halide as the essential component.